Machine vision system and method for estimating and tracking facial pose

ABSTRACT

System and method for estimating and tracking an orientation of a user&#39;s face by combining head tracking and face detection techniques. The orientation of the face, or facial pose, can be expressed in terms of pitch, roll and yaw of the user&#39;s head. Facial pose information can be used, for example, to ascertain in which direction the user is looking. In general, the facial pose estimation method obtains a position of the head and a position of the face and compares the two to obtain the facial pose. In particular, a camera is used to obtain an image containing a user&#39;s head. Any movement of the user&#39;s head is tracked and the head position is determined. A face then is detected on the head and the face position is determined. The head and face positions then are compared.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. Ser. No.11/106,862, entitled “Machine Vision System and Method for Estimatingand Tracking Facial Pose,” filed Apr. 16, 2005, which is now allowed,which is a divisional application of U.S. Pat. No. 6,937,745, entitled“Machine Vision System and Method for Estimating and Tracking FacialPose,” issued Aug. 30, 2005, whereby the entire contents of each ofthese documents is hereby incorporated by reference.

BACKGROUND

Traditional interaction between a user and a computer occurs with thecomputer waiting passively for the user to dictate its actions. Throughinput devices, such as a keyboard and a mouse, the user communicatesactions and intentions to the computer. Although this one-sidedinteraction is common it fails to fully exploit the capabilities of thecomputer.

It is desirable to have the computer play a more active role ininteracting with the user rather than merely acting as a passiveinformation source. A more interactive design involves linking thecomputer to a video camera so that the computer can interact with theuser. The computer achieves this interaction by detecting the presenceof and tracking the user. The user's face in particular providesimportant indications of where the user's attention is focused. Once thecomputer is aware of where the user's is looking this information can beused to determine the user's actions and intentions and reactaccordingly.

An important way in which a computer determines where a user's attentionis focused by determining the facial pose of the user. A facial pose isthe orientation of the user's face. The facial pose can be described interms of rotation about three axes, namely, pitch, roll and yaw.Typically, the pitch is the movement of the head up and down, the yaw isthe movement of the head left and right, and the roll is the movement ofthe head from side to side.

Determining a user's facial pose in real time, however, presents manychallenges. First, the user's head must be detected and tracked todetermine the location of the head. One problem with current real-timehead tracking techniques, however, is that these techniques often areconfused by waving hands or changing illumination. In addition,techniques that track only faces do not run at realistic camera framerates or do not succeed in real-world environments, Moreover, headtracking techniques that use visual processing modalities may work wellin certain situations but fail in others, depending on the nature of thescene being processed. Current visual modalities, used singularly, arenot discriminating enough to detect and track a head robustly. Color,for example, changes with shifts in illumination, and people move indifferent ways. In contrast, “skin color” is not restricted to skin, norare people the only moving objects in the scene being analyzed.

SUMMARY

This Summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This Summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used to limit the scope of the claimed subject matter.

The facial pose estimation system and method disclosed herein includesproviding real-time tracking of and information about a user's facialpose. The facial pose of the user is a position and orientation in spaceof the user's face and can be expressed in terms of pitch, roll and yawof the user's head. Facial pose information can be used, for example, toascertain in which direction the user is looking and consequently wherethe user's attention is focused.

The facial pose estimation system and method of the present inventionprovides at least one advantage over existing techniques. In particular,the facial pose of a user can be synthesized from any combination of:(1) a head-tracking component; and (2) a frontal face-detectingcomponent. The method of the present invention includes using a camerato obtain an image containing a user's head. Next, any movement of theuser's head is tracked and a position of the user's head is determined.A face then is detected on the head and a face position is determined.The head and face positions are then compared to each other to obtainthe facial pose.

The comparison of the head and face positions may be achieved by usingone of at least two techniques. A first technique involves determining acenter of the user's head and constructing a head line between the headcenter and the center of the camera. Next, a face on the head isdetected and the center of the face is computed. A face line isconstructed between the camera center and the face center. A deviationangle is defined as the angle between the head line and the face line.By comparing the deviation angle to a threshold angle, the facial posecan be determined. Alternatively, instead of finding the center of thehead and the center of the face, the centroid of the head and thecentroid of the face may be found and used.

Another technique for comparing the head and face positions involvesobtaining an image containing the user's head and face and finding theface center. A center line is defined as a line that bisects the user'shead into two equal parts. The distance in pixels between the facecenter and the center line is found and compared to a threshold value.The facial pose can be determined by the amount of divergence. Inaddition, if there is divergence of more than the threshold value, thenit may be assumed that the user's attention is not focused on aparticular monitor. On the other hand, if the divergence is less thatthe threshold value, then it may be assumed that the user's attention isfocused on the monitor.

The system of the present invention utilizes a combination of ahead-tracking component in the form of a head tracker, and a frontalface detecting component in the form of a face detection system. Thehead tracker is used to detect and track a user's head and to determinethe position and center of the head. The face detection system is usedto detect a face on the head and to determine the position and center ofthe face. A position comparator compares the head position and the faceposition in accordance with the above method to synthesize the user'sfacial pose.

It should be noted that alternative embodiments are possible, and thatsteps and elements discussed herein may be changed, added, oreliminated, depending on the particular embodiment. These alternativeembodiments include alternative steps and alternative elements that maybe used, and structural changes that may be made, without departing fromthe scope of the invention.

DRAWINGS DESCRIPTION

Referring now to the drawings in which like reference numbers representcorresponding parts throughout:

FIG. 1 is a block diagram illustrating an overview of the facial poseestimation system of the present invention.

FIG. 2A is an illustration of one possible implementation of the facialpose estimation system shown in FIG. 1 where a user is looking at amonitor.

FIG. 2B is an illustration of one possible implementation of the facialpose estimation system shown in FIG. 2A where a user is looking awayfrom the monitor.

FIG. 3 is a block diagram illustrating a computing apparatus suitablefor carrying out the invention.

FIG. 4 is a general flow diagram illustrating the operation of thefacial pose estimation system shown in FIGS. 1, 2A and 2B.

FIGS. 5A, 5B and 5C are general block diagrams illustrating how the headand face positions may be compared to each other.

FIG. 6 is a block diagram illustrating the components of the facial poseestimation system shown in FIGS. 1, 2A and 2B.

FIG. 7 is a flow diagram illustrating the operational details of thefacial pose estimation method of the present invention.

FIGS. 8A and 8B illustrate the facial pose estimation method of thepresent invention in the yaw direction.

FIGS. 9A and 9B illustrate the facial pose estimation method of thepresent invention in the pitch direction.

FIG. 10 is a block diagram illustrating a working example of the headtracker shown in FIG. 6.

FIG. 11 is a detailed block diagram of the head tracker illustrating atemporal or dynamic Bayesian network.

FIG. 12 is a flow diagram illustrating the general operation of the headtracker.

FIG. 13 is a general block-flow diagram illustrating the face detectionsystem shown in FIG. 6.

FIG. 14 is a detailed block diagram illustrating the hypothesis moduleof the face detection system shown in FIG. 13.

FIG. 15 is a detailed block diagram illustrating the preprocessingmodule of the face detection system shown in FIG. 13.

FIG. 16 is a detailed block diagram illustrating the feature extractionmodule of the face detection system shown in FIG. 13.

FIG. 17 is a detailed block diagram illustrating the feature averagingmodule shown in FIG. 13.

FIG. 18 is a detailed block diagram illustrating the relational templatemodule shown in FIG. 13.

DETAILED DESCRIPTION

In the following description of the facial pose estimation system andmethod, reference is made to the accompanying drawings, which form apart thereof, and in which is shown by way of illustration a specificexample whereby the facial pose estimation system and method may bepracticed. It is to be understood that other embodiments may be utilizedand structural changes may be made without departing from the scope ofthe claimed subject matter.

I. General Overview

The present invention includes a facial pose estimation system andmethod for estimating and tracking an orientation of a user's face (alsocalled a facial pose). Information about where the user's attention isfocused may be synthesized from the user's facial pose. This importantinformation about where the user's attention is focused may be used invaried and diverse ways. For example, a screen saver can be controlledby the present invention, such that the screen saver starts when theuser is not looking at a monitor and stops when the user is looking atthe monitor. The present invention also may be used in amultiple-monitor environment to determine at which monitor the user islooking. In this situation a monitor application running on the computercan use the present invention to determine which monitor the user isobserving so that information may be presented to the user.

Another way in which the present invention may be used is to makeavailable the user's state to others. For example, instant messagingapplications can use the present invention to provide a more accurateindication of whether the user is present at his computer and availableto see the message. Using the present invention, a computer coulddetermine which computation to perform at the present time based onwhether the user is looking at the monitor. For example, if the user isfocused somewhere other than the monitor the computer could performbackground computation. The present invention also may be used by anaudio application to determine whether to run speech recognition on anaudio signal from a microphone. Thus, if the user is facing the monitorand speaking the speech recognition is performed. On the other hand, ifthe user is turned away from the monitor speech recognition ceases.Similarly, lip reading applications may use the present invention notifythe application to read the user's lips when the user is facing themonitor and cease when the user is turned away.

The facial pose estimation system determines facial pose informationusing a combination of a head tracker and a face detector. The headtracker provides information about the position in space of the user'shead. In addition, the head tracker is used to track any movement of theuser's head. Once the user's head has been tracked and its positionfound the face detector is used to detect a face on the head. If a faceis detected, then the position of the face in space is determined. Afacial pose, or the orientation of the face in space, can be estimatedby comparing the head position and the face position. This facial poseinformation can provide vital information about the user, such as wherea user's attention is focused.

FIG. 1 is a block diagram illustrating an overview of the facial poseestimation system of the present invention. The system is used to trackand estimate a facial pose of a head 110. Although the head usually willbe a human head, other situations are possible. By way of example, thehead 110 may be a robotic head that is crafted to approximate the lookof a human head. The head 110 usually includes such facial features astwo eyes, a nose and a mouth, but other facial features such as facialhair are possible.

A camera 120 is used to capture visual information 115. In one aspect ofthe invention, the camera 120 captures visual information 115 about thehead 110 in real time. In another aspect of the invention, the head 110is contained in an image or series of images (such as a photograph orvideo sequence) and the camera 120 captures visual information 115 fromthe images. The camera 120 outputs 125 a captured image 130 thatcontains the visual information 115 about the head 110.

The captured image 130 is transmitted 135 to a computing apparatus 140containing a facial pose estimation system 150. The computing apparatus140 may be any device that contains a processor and is capable ofexecuting computer-readable instructions. In one aspect of the inventionthe facial pose estimation system 150 is a software module containingcomputer executable instructions. As described in detail below, thefacial pose estimation system 150 tracks and processes the image 130 inreal time. In addition, the system 150 provides an estimate of thefacial pose 160 of the head 110.

FIGS. 2A and 2B illustrate one type of implementation of the facial poseestimation system 150 of the present invention. In this implementation,the facial pose estimation system 150 is implemented into an attentiondetection system 200. The attention detection system 200 is used tosynthesize information about where a user's attention is focused. Inthis implementation, the camera 120 is located on a monitor 220 and thefacial pose estimation system 150 is used to determine whether a user210 is looking at (or paying attention to) the monitor 220. It should benoted that several other implementations are possible and FIGS. 2A and2B illustrate only a single possible implementation.

Referring to FIG. 2A, the facial pose estimation system 150 isimplemented in an attention detection system 200. The attentiondetection system 200 includes the user 210 sitting in a chair 215 andobserving the monitor 220 that is located on a table 230. The monitor220 provides information to the user 210 and serves as an interfacebetween a personal computer 240 and the user 210.

The facial pose estimation system 150 includes the camera 120 that islocated on the monitor 220. At this location, the camera 120 is capableof observing the user 210, especially the head 110 and face 250 of theuser 210. The camera 120 captures visual information 115 of the user 210and transmits the image 130 to the personal computer 240 for processing.The personal computer 240 includes an input/output interface 260 forallowing devices to be connected to the personal computer 240. Thecamera 120 and the monitor 220 are connected to the personal computervia the input/output interface 260. At least one processor 270 islocated on the personal computer 240 to provide processing capability.The facial pose estimation system 150 and at least one application 280also are located on the personal computer 240.

The facial pose estimation system 150 provides facial pose informationto the attention detection system 200 as follows. The user 210 uses thepersonal computer 240 by sitting in the chair 215 and facing the monitor220. The camera 120 captures at least the head 110 of the user 210 andsends the image 130 of the head 110 to the facial pose estimation system150. The facial pose estimation system 150 receives the image 130through the input/output interface 260 and processes the image 130 usingthe processor 270. The facial pose estimation system 150 determinesfacial pose information in real time and makes the information availableto the application 280.

In this implementation the application 280 uses the facial poseinformation to determine whether the user's 210 attention is focused onthe monitor 220. In other words, whether the user 210 is observing themonitor 220. Depending on the type of application, this facial poseinformation allows the application 280 to determine a good time performan action. By way of example, if the application 280 is an e-mailapplication then the application will to notify the user 210 that he hasan e-mail when the facial pose estimation system 150 determines that theuser 210 is facing the monitor 220.

As shown in FIG. 2A, the user 210 is facing the monitor 220. In thisexample, the facial pose estimation system 150 provides this informationto the application 280 and the application 280 then sends an e-mailnotification message to the monitor 220 knowing that the user 210 islooking at the monitor and will likely see the message.

On the other hand, in FIG. 2B the user 210 is not facing the monitor(because the face 250 of the user 210 is looking away from the monitor220). In this situation the facial pose estimation system 150 determinesthat the facial pose of the user 210 is away from the monitor 220. Thisfacial pose information is reported to the application 280. Using theabove example, the application 280 uses this information and does notsend an e-mail notification message to the monitor 220 because the user210 most likely will not see the message. Instead the application 280waits until the user 210 is facing the monitor 220 to send the message.

II. Exemplary Operating Environment

The facial pose estimation system 150 of the present invention isdesigned to operate in a computing environment. In FIG. 1, the computingenvironment includes a computing apparatus 140 and in FIGS. 2A and 2Bthe computing environment includes a personal computer 240. The followdiscussion is intended to provide a brief, general description of asuitable computing environment in which the invention may beimplemented.

FIG. 3 is a block diagram illustrating a computing apparatus suitablefor carrying out the invention. Although not required, the inventionwill be described in the general context of computer-executableinstructions, such as program modules, being executed by a computer.Generally, program modules include routines, programs, objects,components, data structures, etc. that perform particular tasks orimplement particular abstract data types. Moreover, those skilled in theart will appreciate that the invention may be practiced with a varietyof computer system configurations, including personal computers, servercomputers, hand-held devices, multiprocessor systems,microprocessor-based or programmable consumer electronics, network PCs,minicomputers, mainframe computers, and the like. The invention may alsobe practiced in distributed computing environments where tasks areperformed by remote processing devices that are linked through acommunications network. In a distributed computing environment, programmodules may be located on both local and remote computer storage mediaincluding memory storage devices.

With reference to FIG. 3, an exemplary system for implementing theinvention includes a general-purpose computing device in the form of theconventional personal computer 240 shown in FIGS. 2A and 2B. FIG. 3illustrates details of the computer 240. In particular, the computer 240includes the processing unit 270, a system memory 304, and a system bus306 that couples various system components including the system memory304 to the processing unit 270. The system bus 306 may be any of severaltypes of bus structures including a memory bus or memory controller, aperipheral bus, and a local bus using any of a variety of busarchitectures. The system memory includes read only memory (ROM) 310 andrandom access memory (RAM) 312. A basic input/output system (BIOS) 314,containing the basic routines that help to transfer information betweenelements within the personal computer 240, such as during start-up, isstored in ROM 310. The personal computer 240 further includes a harddisk drive 316 for reading from and writing to a hard disk, not shown, amagnetic disk drive 318 for reading from or writing to a removablemagnetic disk 320, and an optical disk drive 322 for reading from orwriting to a removable optical disk 324 such as a CD-ROM or otheroptical media. The hard disk drive 316, magnetic disk drive 328 andoptical disk drive 322 are connected to the system bus 306 by a harddisk drive interface 326, a magnetic disk drive interface 328 and anoptical disk drive interface 330, respectively. The drives and theirassociated computer-readable media provide nonvolatile storage ofcomputer readable instructions, data structures, program modules andother data for the personal computer 240.

Although the exemplary environment described herein employs a hard disk,a removable magnetic disk 320 and a removable optical disk 324, itshould be appreciated by those skilled in the art that other types ofcomputer readable media that can store data that is accessible by acomputer, such as magnetic cassettes, flash memory cards, digital videodisks, Bernoulli cartridges, random access memories (RAMs), read-onlymemories (ROMs), and the like, may also be used in the exemplaryoperating environment.

A number of program modules may be stored on the hard disk, magneticdisk 320, optical disk 324, ROM 310 or RAM 312, including an operatingsystem 332, one or more application programs 334, other program modules336 (such as the facial pose estimation system 150) and program data338. A user (not shown) may enter commands and information into thepersonal computer 240 through input devices such as a keyboard 340 and apointing device 342. In addition, a camera 343 (such as a video camera)may be connected to the personal computer 240 as well as other inputdevices (not shown) including, for example, a microphone, joystick, gamepad, satellite dish, scanner, or the like. These other input devices areoften connected to the processing unit 270 through a serial portinterface 344 that is coupled to the system bus 306, but may beconnected by other interfaces, such as a parallel port, a game port or auniversal serial bus (USB). The monitor 220 (or other type of displaydevice) is also connected to the system bus 306 via an interface, suchas a video adapter 348. In addition to the monitor 346, personalcomputers typically include other peripheral output devices (not shown),such as speakers and printers.

The personal computer 240 may operate in a networked environment usinglogical connections to one or more remote computers, such as a remotecomputer 350. The remote computer 350 may be another personal computer,a server, a router, a network PC, a peer device or other common networknode, and typically includes many or all of the elements described aboverelative to the personal computer 240, although only a memory storagedevice 352 has been illustrated in FIG. 3. The logical connectionsdepicted in FIG. 3 include a local area network (LAN) 354 and a widearea network (WAN) 356. Such networking environments are commonplace inoffices, enterprise-wide computer networks, intranets and the Internet.

When used in a LAN networking environment, the personal computer 240 isconnected to the local network 354 through a network interface oradapter 358. When used in a WAN networking environment, the personalcomputer 240 typically includes a modem 360 or other means forestablishing communications over the wide area network 356, such as theInternet. The modem 360, which may be internal or external, is connectedto the system bus 306 via the serial port interface 344. In a networkedenvironment, program modules depicted relative to the personal computer240, or portions thereof, may be stored in the remote memory storagedevice 352. It will be appreciated that the network connections shownare exemplary and other means of establishing a communications linkbetween the computers may be used.

III. Operational and System Overview

FIG. 4 is a general flow diagram illustrating the operation of thefacial pose estimation system 150 shown in FIGS. 1, 2A and 2B. Ingeneral, the system 150 analyzes a user's head and face and determinesthe direction that the user is facing. Specifically, the system 150first tracks any movement of the head within the range of a camera (box400). This head tracking ensures that the system 150 will be analyzingthe head. Any one of several head tracking techniques may be used withthe present invention. One head tracking technique using multiplesensing modalities was used in the working example below and isdescribed in detail in Appendix “A”.

Using the camera, the system 150 obtains an image containing the head(box 410). From this image a position of the head is determined (box420). The position of the head may be expressed is several differentways, such as relative to a point in space or a point on an object. Byway of example, the head position may be expressed relative to thecenter of the camera.

Once the head position is established the system 150 performs facedetection to detect a face on the head (box 430). If a face is detectedon the head then the position of the face is determined (box 440). Anyof several face detection techniques may be used with the presentinvention. A face detection technique using a relational template and anon-intensity image property was used in the working example below andis described in detail in Appendix “B”.

As with the head position, the face position may be expressed in avariety of ways, such as relative to the camera center. Next, the headposition and the face position are compared to each other to determine afacial pose (box 450). The facial pose gives indications as to thedirection that the user's face is pointing.

FIGS. 5A, 5B and 5C are general block diagrams illustrating how the headand face positions may be compared to each other to determine the facialpose. FIGS. 5A, 5B and 5C represent a plan view of the facial poseestimation system 150 and the attention detection system 200 shown inFIGS. 2A and 2B. FIGS. 5A, 5B and 5C illustrate the user's head 110 andpart of the user's face. The face 250 is represented by a pair of eyes500 and a nose 510. The head 110 and face 250 are captured by the camera120 that is mounted on the monitor 220.

The facial pose estimation system 150 determines a facial pose bycomparing a head position and a face position. The head position isrepresented in FIGS. 5A, 5B and 5C by a head position bar 530. The headposition bar 530 represents the portion of the camera view that thesystem 150 recognizes as the head 110. Likewise, the position of theface 250 is represented by a face position bar 540. The face positionbar 540 represents the portion of the camera view that the system 150recognizes as the face 250.

The head position bar 530 is bisected by head position bar line 542 thatis an imaginary line (shown as a short dashed line in FIGS. 5A, 5B and5C) from the center of the camera 120 through the center of the headposition bar 530. Similarly, the face position bar 540 is bisected by aface position bar line 544 (shown as a long dashed line in FIGS. 5A, 5Band 5C) from the center of the camera 120 through the center of the faceposition bar 540.

The facial pose estimation system 150 determines facial pose bycomparing the angle 546 between the head position bar line 542 and theface position bar line 544. By determining the angle between theposition bar lines 542, 544, the system 150 can estimate the directionthe user's face 250 is pointed and thus the facial pose.

Referring to FIG. 5A, the user's face 250 is facing in the direction ofthe monitor 220. The facial pose estimation system 150 determines thisby noting that that the head position bar line 542 and the face positionbar line 544 are lined up with each other and at little or no angle. Inthis situation as shown in FIG. 5A, the user's face 250 is lookingforward toward the camera 120 and monitor 220. A first arrow 550 showsthe direction that the user's face 250 is looking.

In FIG. 5B, the angle 546 between the head position bar line 542 and theface position bar line 540 is larger than in FIG. 5A. This means thatthe user's face 250 is pointed slightly away from the camera 120 andmonitor 220. A second arrow 560 shown that the user's face 250 islooking slight away from the camera 120 and monitor. As discussed indetail below, in some implementations of the invention the angle 546 iscompared to a certain threshold angle. If the angle 546 is greater thanthe threshold angle, then the user's face 250 is considered pointed awayfrom the camera 120 and monitor 220. On the other hand, if the angle 546is smaller that the threshold angle, then the user's face 250 isconsidered pointed toward the camera 120 and monitor.

In FIG. 5C, the angle 546 between the head position bar line 542 and theface position bar line 5404 is larger than in FIGS. 5A and 5B. In thiscase, the facial estimation system 150 determines that the user's face250 is facing away from the camera 120 and monitor. The direction inwhich the user's face 250 is pointing is shown by the third arrow 570.

The determination that the user is looking at the monitor if the facialpose is within the threshold angle may involve one or more techniques.By way of example, probability may be used. A Gaussian probability curvemay be used to show that if a user's facial pose is within a thresholdangle then there is a high probability that the user is looking at themonitor. Conversely, if the user's facial pose is greater than thethreshold angle there is a high probability that the user is not lookingat the monitor. Eye tracking may also be used to determine whether auser is looking at the monitor. Eye tracking involves tracking theuser's eyes to determine where the user is looking. Typically, eyetracking would be used when the facial pose is less than the thresholdangle.

FIG. 6 is a block diagram illustrating the components of the facial poseestimation system 150 shown in FIGS. 1, 2A and 2B. The facial poseestimation system 150 includes a head tracker 610, a face detectionsystem 620, a position comparator 630, and an output module 640. Thefacial pose estimation system 150 may also include an optional temporalfilter 645 for filtering out any sudden and temporary movements of theuser's face. For example, a temporal filter may be used to filter outthe facial movement when the user's looks away for a brief moment to geta pen. This optional temporal filter is shown in FIG. 6 as optional bythe alternating dotted and dashed line.

The image 130 is obtained (such as by using a camera) and thentransmitted to the system 150 for processing. The head tracker 610tracks a head within the image 130 and determines a position of the headrelative to a certain point. In most cases the head position will bedetermined relative to the center of the camera 120.

Once the head position is determined, the face detection system 620determines whether the head has a face. If so, then the face detectionsystem 620 determines the position of the face relative to certainpoint, such as the center of the camera 120. The position comparator 630receives the head and face position and, as outlined above and detailedbelow, determines the facial pose by comparing the head and facepositions. Facial pose information is synthesized using this comparisonand this information is sent to the output module 640 for distributionto one or more applications 650.

IV. Operational Details and Working Example

The following working example is used to illustrate the operationaldetails of the invention. This working example includes theimplementation of FIGS. 2A and 2B in which the facial pose estimationsystem 150 is incorporated into the attention detection system 200. Thisworking example is provided as an example of one way in which facialpose estimation system may operate and be used. It should be noted thatthis working example is only one way in which the invention may operateand be used, and is provided for illustrative purposes only.

The comparison of the head and face positions may be achieved by usingone of at least two techniques. The working example presented uses afirst technique outlined above that involves determining a center of theuser's head and constructing a head line between the head center and thecenter of the camera. Next, a face on the head is detected and thecenter of the face is computed. A face line is constructed between thecamera center and the face center. A deviation angle is defined as theangle between the head line and the face line. By comparing thedeviation angle to a threshold angle, the facial pose can be determined.

Another aspect of the present invention includes a comparison techniquethat involves obtaining an image containing the user's head and face andfinding the face center. A center line is defined as a line that bisectsthe user's head into two equal parts. The distance in pixels between theface center and the center line is found and compared to a thresholdvalue. The facial pose can be determined by the amount of divergence. Inaddition, if there is divergence of more than the threshold value, thenit may be assumed that the user is not looking at the monitor in frontof him. On the other hand, if the divergence is less that the thresholdvalue, then it may be assumed that the user is looking at the monitor.

FIG. 7 is a flow diagram illustrating the operational details of anattention detection system 200 using the facial pose estimation system150 and the first comparison technique described above. In particular,the attention detection method begins by obtaining an image containing ahead (box 700). Once the image is obtained, the head tracker 610determines a center of the head in relation to a camera center (box705). Next, a head line is constructed by drawing an imaginary linebetween the camera center and the head center (box 710). These stepsprovide information about the position of the head.

The face detection system 620 is then used to detect a face on the head(box 715). A determination is then made as to whether a face was found(box 720). If a face is not detected, then if may be inferred that theuser is not looking at the camera (box 725). If a face is detected, thenthe face detection system 620 determines a center of the face inrelation to the camera center (box 730). A face line is then constructedby drawing an imaginary line between the camera center and the facecenter (box 735).

The radius of the user's head is then determined (box 740). This may bedone by guessing, by using the head tracker, or by asking the user toinput the radius of his head. In addition, the radius of the user's headmay be determined by knowing the average radius of a human head andusing this knowledge to estimate the radius of the user's head. Next, adeviation angle between the head line and the face line is determined(box 745). A determination is then made as to whether the deviationexceeds a threshold angle (box 750).

If the deviation angle does exceed the threshold angle, then it may beinferred that the user is not looking at the camera (box 725). If thedeviation angle does not exceed the threshold angle, then it may beinferred that the user is looking at the camera (box 755). The thresholdangle depends on the distance from the user to the camera and the sizeof the monitor. The threshold angle may be selected empirically by anoperator of the facial pose estimation system. In this working example,the threshold angle was 30 degrees. Thus, if the deviation angle wasless than 30 degrees the user was considered to be looking at thecamera. Alternatively, if the deviation angle was greater than 30degrees the user was considered to be looking away from the camera.

The deviation angle may determined in at least three directions. Thesedirections include the pitch, roll and yaw of the user's head. In thisworking example, the pitch of the user's head is measure about anx-axis, the roll is measured about a z-axis, and the yaw is measureabout a y-axis. The facial pose estimation method detailed above may beused in any one of these directions. In this working example, only thedeviation angle in the pitch and yaw directions were determined.Deviation in the roll direction tend not to have a large impact onwhether the user is facing the monitor.

FIGS. 8A and 8B illustrate the facial pose estimation method in the yawdirection. As shown by the axes 800, the yaw direction is in the x-zplane, or a plan view of FIGS. 2A and 2B. In FIG. 8A, the yaw of theuser's head is such that the user is observing the monitor 220.Conversely, in FIG. 8B, the yaw of the user's head is such that the useris not observing the monitor 220. The details of how this determinationwas made will now be explained.

In FIG. 8A the user's head 110 is in front of the monitor 220 that hasthe camera 120 located thereon. A center of the camera C_(C) havingcoordinates x-y-z equals coordinates (0,0,0) is determined. Next, acenter of the head C_(H) having coordinates (x_(H), y_(H), z_(H)) and acenter of the face C_(F) having coordinates (x_(F), y_(F), z_(F)) arefound by the head tracker and the face detection system, respectively. Ahead line 810 is drawn from camera center C_(C) to the head centerC_(H). Similarly, a face line 820 is drawn from camera center C_(C) toface center C_(F). As shown in FIG. 8A, the yaw deviation angle is theangle between the head line 810 and the face line 820 in the x-z plane.Mathematically, the yaw deviation angle is found using the equation,Yaw=a sin((x _(F) −x _(H))/r),where r is the radius of the user's head. It should be noted that thisequation is an example and that there are many approximations thatcompute similar values for the yaw deviation angle. In addition,information different from the variables x_(F), x_(H) and r may be usedin which case the yaw deviation would be computed differently.

The yaw deviation angle is then compared to a threshold angle. Thethreshold angle is visualized by drawing a threshold line 830 from thecamera center C_(C) at the threshold angle away from a camera centerline (not shown) that is perpendicular to the front of the camera 120.As seen in FIG. 8A, the yaw deviation angle is less than the thresholdangle. Thus, the facial pose estimation method infers that the user isobserving the monitor 220.

Referring to FIG. 8B, the yaw deviation angle is greater than thethreshold angle. The facial pose estimation method thus assumes that theuser is not observing the monitor 220 because the user's face 250 ispointed away from the monitor 220.

FIGS. 9A and 9B illustrate the facial pose estimation method in thepitch direction. As shown by the axes 900, the pitch direction about thex-axis in the y-z plane. This is the same view as shown in FIGS. 2A and2B. In FIG. 9A, the pitch of the user's head 110 is such that the user210 is observing the monitor 220. On the other hand, in FIG. 9B, thepitch of the user's head 110 is such that the user 210 is not observingthe monitor 220. The details of how this determination was made will nowbe explained.

In FIGS. 9A and 9B, the user 210 is sitting in the chair 215 at thetable 230. The table 230 contains the monitor 220 and the camera 120mounted on top of the monitor 220. The user's head 110 includes facialfeatures that make up the face 250 such as the eyes 500 and the nose510.

Referring to FIG. 9A, the head center C_(H) in x-y-z coordinates isdetermined by the head tracker. Similarly, the face center C_(F) isdetermined by the face detection system. The head line 810 is drawn fromthe camera center C_(C) to the head center C_(H) and the face line 820is drawn from the camera center C_(C) to the face center C_(F). Thepitch deviation angle is the angle between the head line 810 and theface line 820 in the y-z plane. Mathematically, the pitch deviationangle is computed using the equation,Pitch=a sin((y _(F) −y _(H))/r),where r is the radius of the user's head. Once again, it should be notedthat this equation is an example and that there are many approximationsthat compute similar values for the pitch deviation angle. In addition,information different from the variables x_(F), x_(H) and r may be usedin which case the pitch deviation would be computed differently.

The pitch deviation angle is compared to the threshold angle. As shownin FIG. 9A, the pitch deviation angle is less than the threshold angle.Thus, the facial pose estimation method determines that the user 210 isobserving the monitor 220. On the other hand, in FIG. 9B, the pitchdeviation angle is greater than the threshold angle. In this situation,the method determines that the user 210 is not observing the monitor220. This may occur, for example, when the user 210 is looking down at abook or paper in his lap.

The foregoing Detailed Description has been presented for the purposesof illustration and description. Many modifications and variations arepossible in light of the above teaching. It is not intended to beexhaustive or to limit the subject matter described herein to theprecise form disclosed. Although the subject matter has been describedin language specific to structural features and/or methodological acts,it is to be understood that the subject matter defined in the appendedclaims is not necessarily limited to the specific features or actsdescribed above. Rather, the specific features and acts described aboveare disclosed as example forms of implementing the claims appendedhereto.

APPENDIX “A” Details of the Head Tracking System and Method Used in theWorking Example

I. Head Tracking Introduction and Overview

Several different types of head tracking systems and methods may be usedwith the present invention. In the working example presented above, ahead tracking method and system that fuses results of multiple sensingmodalities was used. This head tracking system and method are set forthin co-pending U.S. patent application Ser. No. 09/323,724 by Horvitz etal., filed on Jun. 1, 1999, entitled “A System and Method for TrackingObjects by Fusing Results of Multiple Sensing Modalities”. The detailsof this head tracking system and method as used in this working examplewill now be discussed.

The head tracker used in this working example is a system and method forfusing results of multiple sensing modalities to efficiently performingautomated vision tracking, such as tracking human head movement andfacial movement. FIG. 10 is a general block diagram illustrating anoverview of the head tracker 610 of FIG. 6. The head tracker 610robustly tracks a target object 1008 (such as a user's head 110) byinferring target data 1010, such as the state of the object 1008,including position or object coordinate information, orientation,expression, etc., conditioned on report information 1012 produced by atleast one sensor modality 1014 tracking the target 1008. The headtracker 610 can be used as a vision-based tracking system for trackingobjects of a digitized video scene, such as an input sequence of digitalimages. The input sequence can be from a live camera or from a sequenceof images stored on a recording medium, such as a tape, disk, or anysuitable source medium. The target data 1010 can be true stateinformation about the target object 1008 of the image sequence.Different types of data present in the image sequence, such as color,edge, shape, and motion, can be considered different sensing modalities.

In this case, the head tracker 610 is a Bayesian network for performingBayesian vision modality fusion for multiple sensing modalities. TheBayesian network captures the probabilistic dependencies between thetrue state of the object 1008 being tracked and evidence obtained frommultiple tracking sensing modalities 1014. A Bayesian network is adirected acyclic graph that represents a joint probability distributionfor a set of random variables. As shown in FIG. 10, the Bayesian networkhead tracker 610 includes nodes 1010, 1012, 1016, 1018 and 1020 thatrepresent variables of interest or random variables. Arcs or lineconnectors 1030, 1032 and 1034, 1035 represent probabilisticdependencies among pairs of variables. The Bayesian network facilitatesmaking associative and causal assertions about probabilistic influencesamong the variables.

The head tracker 610 constructs, learns, and performs inference withBayesian models. This includes the use of exact and approximatealgorithms for Bayesian-network inference procedures, methods that allowfor the learning of conditional probabilities represented in a Bayesianmodel, the induction of network structure from data, and networks forreasoning over time. In addition, conceptual links between Bayesiannetworks and probabilistic time-series analysis tools such as hiddenMarkov models (HMMs) and Kalman filters can be implemented in thepresent invention. HMMs and Kalman filters can be represented byBayesian networks with repetitive structure capturing prototypicalpatterns of independence among classes of variables.

II. Components and Operation of a Single Modality of the Head Tracker

For each sensor modality 1014, nodes 1012, 1018 and 1020 are variablesthat are instantiated by the sensor modality 1014 and nodes 1010 and1016 represent inferred values. In particular, node 1010 is a targetground truth node that represents an unknown state of the target objectand the goal of head tracker 610 inference.

From a Bayesian perspective, the ground-truth state influences or causesan output from the sensor modality 1014 (it should be noted that the useof term “causes” comprises both deterministic and stochasticcomponents). This influence is indicated with arc 1030 from the groundtruth node 1010 to the modality report node 1012. The modality reportnode 1012 is also influenced by its reliability, or its ability toaccurately estimate ground-truth state, as indicated with an arc 1032from the modality reliability node 1016 to the modality report node1012.

Although reliabilities themselves typically are not directly observed,both reliabilities and estimates of reliabilities vary with thestructure of the scene being analyzed. To build a coherent framework forfusing reports from multiple modalities, reliability can be consideredas an explicit or implicit variable. From this, probabilistic submodelsare built to dynamically diagnose reliability as a function of easilyascertainable static or dynamic features detected by the automatedanalysis of the image. As shown in FIG. 10, such evidence is representedby n modality reliability indicator nodes 1018, 1020 which are in turninfluenced by the modality reliability node 1016, as indicated by thearcs 1034, 1035.

During operation for a single modality, the Bayesian model isinstantiated with the modality report 1012 and reliability indicators1018, 1020 associated with the sensor modality 1014. It should be notedthat the order or frequency that the modality contributes its report isflexible. The reliability of the sensor modality 1014 is computed andthe modality report 1012 is used to provide a probability distributionover the ground-truth state 1010 of the target object 1008. The Bayesiannetwork head tracker 610 is equivalent to the following statement ofconditional probabilities (for simplicity of illustration, n=1):P(t, m, r, i)=P(t)P(m|t, r)P(r)P(i|r)  (1)With this, it can be shown that, for example, the probability densityfor the estimate of the ground-truth state depends both upon the reportas well as the reliability indicator. If t and i were independent, then:P(t, i|m)=P(t|m)P(i|m).However, $\begin{matrix}{{P\text{(}t},{{i\left. m \right)} = {\frac{\int{{P\left( {t,m,r,i} \right)}{\mathbb{d}r}}}{P(m)} = {{P\left( t \right.}m\text{)}{\int{{P\left( {r\left. {t,m} \right){P\left( i \right.}r} \right)}{\mathbb{d}r}}}}}},{and}} & (2) \\{\left. {{P\left( {t\left. m \right){P\left( i \right.}m} \right)} = {{P\left( t \right.}m}} \right){\int{{P\left( r \right.}m\text{)}{P\left( {i{{r{\mathbb{d}r}}}} \right.}}}} & (3)\end{matrix}$Thus, in general, t and i would be independent only if P(r|m)=P(r|t, m).Similarly, however, this would only be true if P(m|t, r)=P(m|t), whichmay violate the assumption that the report, m is conditionally dependenton both ground-truth state, t and reliability, r.

Further, given the conditional probabilities that appear on the righthand side of Equation (1), the probability density for ground-truthstate can be computed, given a report and reliability indicators:$\begin{matrix}{{P\text{(}t\left. {m,i} \right)} = \frac{\int{{P(t)}{P\left( {m\left. {t,r} \right){P(r)}{P\left( {i\left. r \right){\mathbb{d}r}} \right.}} \right.}}}{\int{\int{{P(t)}{P\left( {m\left. {t,r} \right){P(r)}{P\left( {i\left. r \right){\mathbb{d}r}{\mathbb{d}t}} \right.}} \right.}}}}} & (4)\end{matrix}$III. Fusion of Multiple Modalities of the Head Tracker

In the description above for FIG. 10, a model for inferring theprobability distribution over the true state of a target was consideredfrom a report by a single modality. FIG. 11 is a detailed block diagramillustrating a temporal or dynamic network model 1100 capturing temporaldependencies among variables at adjacent points in time for integratingmultiple modalities for tracking at least one object, such as an objectsimilar to object 1008 of FIG. 10, in accordance with the presentinvention.

The network 1100 includes multiple ground truth states 1110, 1112 eachhaving associated multiple modalities 1114, 1116, respectively. Eachmodality 1114, 1116 produces a modality report represented by nodes1122, 1124, 1126, 1128 respectively, influenced by correspondingmodality reliability nodes 1130, 1132, 1134, 1136. Evidence representedby respective 1 through n modality reliability indicator nodes1138-1140, 1142-1144, 1146-1148, 1150-1152 is in turn caused orinfluenced by respective modality reliability nodes 1130, 1132, 1134,1136.

The temporal network 1100 of FIG. 11 extends the single modalityembodiment of FIG. 10 in two ways. First, the network 1100 of FIG. 11includes subsequent ground truth states, t_(n), and multiple modalities1114, 1116, namely sensor modalities A and B for the subsequent groundtruth states t_(n) 1112. Each modality 1114, 1116 produces subsequentmodality reports 1124, 1128 (reports A and B) influenced by respectivereliability submodels 1132, 1136 (submodels A and B) for the subsequentground truth states t_(n) 1112. It should be noted that although twomodalities and respective reports and reliabilities (A and B) are shownin FIG. 11, m different modalities can be included in a similar manner.

The model is further extended to consider temporal dynamics, as well. Inthe simplest approach, the reliability indicator nodes 1138 and 1140,1142 and 1144, 1146 and 1148, 1150 and 1152 can be defined as functionsof the dynamics of image features. For example, for image sequences,rapid change in global intensity values over the image could be used asan indicator variable.

In a more explicit approach, the model 1100 can be extended so that setsof variables are labeled as states at different times. Representationsof Bayesian networks over time that include temporal dependencies amongsome subset of variables are referred to as dynamic Bayesian networks.In the model of FIG. 11, a previous true state directly influences acurrent true state and where prior reliability indicators influencecurrent indicators. For example, as shown in FIG. 11, previous groundtruth t_(n−1) (node 1110) directly influences a current ground trutht_(n) (node 1112) and where prior reliability indicators (nodes 1138 and1148) influence current indicators (nodes 1142 and 1152). By modelingthe integration of multiple modalities and considering the changingreliabilities of reports, a flexible filter is gained which weightsprevious estimates to different degrees based on estimates of theiraccuracy.

IV. Operation of the Head Tracker

FIG. 12 is a block/flow diagram illustrating the general operation ofthe head tracker 610. In general, for video scenes and imageapplications, new digital image data relating to a target object isfirst received by the head tracker 610 from, for instance, a live cameraor storage (box 1200). A modality processor 1212 comprised of multiplevision sensing modalities receives the new digital image data. Themodality processor 1212 computes some or all of estimates andreliability indicators for each modality. Specifically, the modalityprocessor 1212 can estimate states using modalities 1, 2 . . . n (boxes1214-1218) and compute reliability indicators for modalities 1, 2 . . .n (boxes 1220-1224). Next, a sensor fusion analysis processor receives1226 the estimate and reliability indicator computations and infersstates using Bayesian inference (box 1228). Last, a state estimate isproduced that is a synthesized assessment of the computations (box1230).

Referring to FIG. 11 along with FIG. 12, during operation, the modelsfor Bayesian modality fusion are instantiated with reports 1122-1128 andreliability indicators 1138-1152, as shown in FIG. 11. The reliability1130-1136 of each modality is computed by the processor 1212 and thereports 1122-1128 from the modalities are integrated to provide aprobability distribution over the ground-truth state of the targetobject.

Further, the Bayesian network of the head tracker 610 can be trained onreal data to assess the probabilities of the effects of indicators onmodality reports. In addition, reports could be biased based on changinginformation related to the modalities.

APPENDIX “B” Details of the Face Detection System and Method Using inthe Working Example

I. Face Detection Introduction and Overview

Many types of face detection systems and methods may be used with thepresent invention. In this working example, a face detection system andmethod uses a relational template over a geometric distribution of anon-intensity image property was used. This face detection system andmethod are set forth in co-pending U.S. patent application Ser. No.09/430,560 by K. Toyama, filed on Oct. 29, 1999, entitled “A System andMethod for Face Detection Through Geometric Distribution of aNon-Intensity Image Property”. The details of this face detection systemand method as used in this working example will now be discussed.

The face detection system and method using in this working example,preprocesses a cropped input image by resizing to some canonical imagesize, uses a texture template sensitive to high spatial frequencies overthe resized image, averages the pixels comprising each facial feature,and outputs the results of a relational template. A face is detected ifthe output from the relational template is greater than an empiricallydetermined threshold. In this working example, the non-intensity imageproperty used is edge density, which is independent of both person andillumination. The face detection system and method was used first on anentire raw image (so that the cropped image was defined as the entireraw image). Next, smaller sub-regions were defined and searched usingthe face detection system and method. These sub-regions were defined fora limited range of scales that included only those scales on which aface would be located if the user was sitting in front of a desktopcomputer. The face detection method, however was performed over theentire image, for every hypothesized rectangle in which a face couldappear.

FIG. 13 is a general block-flow diagram illustrating the face detectionsystem 620 shown in FIG. 6. Generally, the face detection system 620 ofinputs an image to be examined, determines a sub-region of the image toexamine, performs preprocessing on the sub-region, performs featureextraction based on image property and uses a relational template todetermine if a face is present in the sub-region. The image 130 isreceived by the face detection system 620 and sent to a hypothesismodule 1300 that generates a hypothesis and defines the dimensions of asub-region in the image 130 (or cropped image) where a face may befound. The cropped image is sent as output (box 1310) to a preprocessingmodule 1320, which prepares the image 130 for further processing. Thepreprocessed cropped image is then sent to a feature extraction module1330.

The feature extraction module 1330 extracts any facial features presentin the preprocessed cropped image by using a feature template based onan image property. Further, image features values are obtained by thefeature extraction module 1330 and sent to a feature averaging module1340. The feature averaging module 1340 determines a number of facialregions, places the image features values into a facial regions anddetermines a combined image feature value for each facial region. Thecombined values are then sent to a relational template module 1350 thatbuilds a relational table and determines a relational value based oneach region's combined image feature value.

Based a comparison between the relational value and a threshold value,the face detection system 620 determines whether a face has beendetected in the cropped image (box 1360). If not, then a face is notwithin in the sub-region that was examined and a different sub-regionneeds to be generated (box 1370). This occurs by returning to thehypothesis module 1300 where a different hypothesis is generated aboutwhere a face may be located within the image 130. In addition, based onthe hypothesis generated a different cropped image is defined forexamination as described previously. If a face is detected in thecropped image then face information is sent as output (box 1380). Faceinformation includes, for example, an image of the face, the location ofthe face within the image 130, and the location and dimensions of thecropped image where the face was found.

II. Face Detection System and Operational Details

FIG. 13 is a detailed block diagram illustrating the hypothesis moduleof the face detection system 620 shown in FIGS. 6 and 13. Generally, thehypothesis module 1300 generates an assumption as to the location of aface within the image 130 and defines the dimensions of a sub-region(within the image 130) in which to look for a face. The hypothesismodule 1300 includes a generation module 1400, for generating ahypothesis about where a face may be located, and a cropping module1410, for defining a sub-region to examine.

The generation module 1400 receives the image 130 (box 1420) andgenerates a hypothesis about the location of a face within the image 130(box 1430). The hypothesis may include, for example, information aboutwhich image scales, aspect ratios and locations to examine. In oneaspect of the face detection method, hypotheses are generated thatinclude rectangular sub-regions of the image within a range of scalesand at all possible image locations. Other aspects of the inventioninclude hypothesis generation that may include other types of visionprocessing that target regions of the image most likely to contain aface (such as regions of the image that contain skin color orellipse-shaped blobs). The generated hypothesis is then sent as output(box 1440) to the cropping module 1410.

The cropping module 1410 then defines the dimensions and shape of asub-region (or cropped image) based on the generated hypothesis (box1450). The dimensions and shape are applied to the image 130 (box 1460)and a cropped image is sent as output (box 1470). It should be notedthat the dimensions of the sub-region range between a small percentageof the image 130 to the entire image 130. Further, in one aspect of theinvention, the shape of the sub-region is rectangular. Other aspects ofthe invention include sub-regions that may be any suitable shape thatfacilitates detection of a face within the sub-region (such as oval,circular or square). In some embodiments, once the dimensions and shapeof the sub-region are defined, the entire image 130 is searched bycycling each sub-region through the face detection system 620.Examination of each sub-region may occur one sub-region at a time or, ifmultiple processors are available, concurrent examination may beperformed.

FIG. 15 is a detailed block diagram illustrating the preprocessingmodule 1320 of the face detection system 620. The preprocessing module1320 receives the cropped image that may contain a face and performsvarious types of preprocessing. This preprocessing includes resizing theimage, masking the image to filter out unwanted background noise,performing histogram equalization on the image, or any other type ofpreprocessing that will enhance the raw image for further processing bythe face detection system 620.

In general, the preprocessing module 1320 can include several types ofmodules for performing the preprocessing listed above. In a someembodiments, the preprocessing module includes a resizing module 1500for resizing the cropped image. Moreover, an equalization module 1508for increasing image contrast may optionally be included in someembodiments (as shown by the large dashed line around the equalizationmodule 1508 in FIG. 15). It should be noted that processing of thecropped image by these modules may occur in any suitable order. In thefollowing description, however, the resizing module 1500 is discussedfirst.

The resizing module 1500 resizes the cropped image to an optimal (orcanonical) size using such methods as, for example, smoothing,downsampling and pixel interpolation. This resizing reduces the effectsof image resolution and scale that can substantially change qualities ofan image. The resizing module 1500 shown in FIG. 15 uses pixelinterpolation, but it should be understood that any other suitablemethod of resizing an image (such as those listed above) may be used. Inone aspect of the invention, the resizing module 1500 begins processinga cropped image by determining the actual dimensions (such as horizontaland vertical) of the image (box 1516). In addition, a set of optimaldimensions for the image is selected (box 1524). A comparison then ismade to determine whether the actual dimensions are less than theoptimal dimensions (box 1532). If the actual dimensions are less, thenadditional pixels are generated and added to the actual dimensions toachieve the optimal dimensions (box 1540). One aspect of the inventionincludes generating additional pixels using linear (if one dimension istoo small) or bilinear (if both dimensions are too small) interpolation.If the actual dimensions are greater than the optimal dimensions, thenthe actual dimensions are resized to achieve the optimal dimensions (box1548). In some embodiments, this resizing is performed using Gaussiansmoothing and downsampling. A resized image having optimal dimensions isthen sent as output (box 1556).

The optional equalization module 1508 receives the cropped image (box1564) and determines a maximum and a minimum of each pixel's intensityvalue within the cropped image (box 1572). A transformation is appliedto the intensity value of each pixel (box 1580) and the transformedpixel intensity values placed back into the image (box 1588). In someembodiments, this transform is a histogram equalization that applies alinear transformation on each pixel intensity value in the image, suchthat the resulting image spans the full range of grayscale values. Forexample, each pixel value p is transformed to p′=ap+b, where a and b arechosen so that one of the pixels assumes the maximum possible grayscalevalue while another pixel assumes the minimum value, and all others fallin between. The values for a and b are held constant for any given inputimage. After all pixels are transformed, the resulting contrast-enhancedimage is sent as output (box 1596).

FIG. 16 is a detailed block diagram illustrating the feature extractionmodule 1330 of the face detection system 620. The feature extractionmodule 1330 uses a non-intensity image property to detect local featurespresent in the image. The non-intensity image property is used in afeature template that in some embodiments is sensitive to high spatialfrequencies. A cropped image is received as input (box 1600) and, foreach pixel within the cropped image, image feature values based on thenon-intensity image property are extracted (box 1610) and sent as output(box 1620). The image feature values are extracted by using the featuretemplate to determine the degree of high-frequency variation that occursaround each pixel. In this working example, the image property is edgedensity. Edge density is the amount of local high-frequency texturewithin an area of the face. For example, high edge density is normallyfound around the eyes, where facial features such as the limbus, theeyelids and the eyelashes project several edges onto the image. Incontrast, areas of the face such as the cheeks contain few edges andthus have low edge density. This low edge density occurs whether thecheeks are smooth shaven or covered by facial hair.

One aspect of the invention includes using convolution to convolve thepreprocessed image with at least one feature template based on edgedensity (known as a texture template). The output of the convolution ishigh in areas where there are many edges and low in areas where thereare not. In some embodiments, edge detection is performed using an edgemask (such as a 1, 0, −1 edge mask) applied both horizontally andvertically. For each pixel, the extracted information includes a maximumvalue of the absolute values of each respective convolution.Alternatively, other means of extracting image property information froman image (i.e. feature templates) other than convolution may be used,such as, for example, Laplacians, Gabor wavelets, and any other types offilters than can act as detectors of high-frequency components in animage.

FIG. 17 is a detailed block diagram illustrating the feature averagingmodule 1340 shown in FIG. 13. The feature averaging module 1340 definesfacial regions and combines (e.g., averages or otherwise aggregates andsummarizes) the image feature values within each facial region. In someembodiments, each facial region corresponds to a feature on a face andthe facial regions are geometrically distributed in a facial arrangement(i.e., according to how features of a face are arranged). For example, aforehead region would be above a right eye region and a left eye regionand a mouth region would be below a nose region. In addition, the numberof facial regions can be any number including one or greater. Forexample, in one embodiment the number of facial features is seven,corresponding to forehead, right eye, left eye, right cheek, left cheek,nose and mouth regions.

The feature averaging module 1340 inputs the image feature values (box1700) and defines facial regions (box 1710). The image feature valuesare then grouped into corresponding facial regions (box 1720) and all ofthe image property values for each facial region are combined (box1730). In some embodiments, the image property values for each facialregion are averaged. For instance, if the image property is edge densityand there are eighteen pixels within a right eye region, that regionmight be represented by an average texturedness value of the eighteenpixels. A combined image feature value for each facial region is sent asoutput (box 1740).

FIG. 18 is a detailed block diagram illustrating the relational templatemodule 1350 shown in FIG. 13. In general, the relational template module1350 determines the relationship between any two facial regions andassigns a regional value based on that relationship. Regional values arethen summed to yield a relational value and, if the relational value isgreater than a threshold, a face has been detected. Specifically, therelational template module 1350 inputs the facial regions and combinedimage feature values (box 1800) from the feature averaging module 1340.Two facial regions of interest are selected (box 1808) and, using arelational template, a relationship is determined between the two facialregions (box 1816). The relational template is generally a matrix thatis fixed throughout the face detection operation. The relationaltemplate module 1350 then determines whether the relationship betweenthe two facial regions is satisfied (box 1824). For instance, arelationship may be that a forehead region must have a lower edgedensity than a left eye region.

If the relationship is satisfied, a “true” regional value is defined(box 1832); otherwise, a “false” regional value is defined (box 1840).By way of example, if the forehead region has a lower edge density thanthe left eye region the relationship is satisfied and the regional valuewould be +1 (or “true”). Otherwise, the regional value would be −1 (or“false”). The regional value associated with the relationship betweenthe two facial regions is then stored (box 1848). The relationaltemplate module 1350 then determines whether all of the facial regionsof interest have been examined (box 1856). If all the regions have notbeen examined, the relationship between two different facial regions isexamined. Otherwise, a relational value is determined using the storedregional values (box 1864). In some embodiments, the relational value isdetermined by summing the regional values. For example, if fiverelationships are satisfied (+1*5=5) and two relationships are notsatisfied (−1*2=−2) the relational value would be equal to three(5+(−2)=3).

The relational value is sent as output (box 1872) to be compared to athreshold value (see FIG. 13). If the relational value is greater than acertain empirically-determined threshold value then a face has beendetected within the image. In particular, a face is detected if:${\sum\limits_{i,j}{{{sgn}\left( {I_{i} - I_{j}} \right)}t_{ij}}} > y$where sgn(I_(i)-I_(j)) returns a +1, 0, or −1 depending on the sign ofits argument, y is a threshold determined empirically, and the sum istaken over all possible values of i and j where i<j. In addition, anytype of postprocessing directed at improving speed and eliminatingredundancies may be performed on the face image at this point. Forexample, if two faces are detected and overlap by more than a certainamount then post processing would determine that the two overlappingfaces were really one face and merge the two faces into one.

1. An attention detection system for determining whether a computerapplication should execute an action, comprising: an image captured by acamera that contains the user's head and face; a display device forproviding visual information to the user; a yaw center line defined as aline in an x-z yaw plane that bisects the user's head into twoapproximately equal parts; and a facial pose estimation system thatprocesses the image and uses the yaw center line to determine whether auser is gazing at the display device and instructs the computerapplication whether to execute an action based on the determination. 2.The attention detection system of claim 1, further comprising a headtracker that tracks a head position of the user's head in the image. 3.The attention detection system of claim 2, further comprising a facedetection system that detects a face position of the user's face in theimage.
 4. The attention detection system of claim 3, further comprisinga position comparator that receives the head position and the faceposition and determines a facial pose of the user by comparing the headand face positions.
 5. The attention detection system of claim 1,further comprising a deviation angle in an x-z yaw plane that indicatesa yaw angle between a center of the camera and a center of the user'sface.
 6. The attention detection system of claim 5, further comprising athreshold angle in the x-z yaw plane that is compared to the deviationangle to determine whether the user is facing the display device.
 7. Theattention detection system of claim 1, further comprising a deviationangle in a y-z pitch plane that indicates a pitch angle between a centerof the camera and a center of the user's face.
 8. The attentiondetection system of claim 7, further comprising a threshold angle in they-z pitch plane that is compared to the deviation angle to determinewhether the user is facing the display device.
 9. The attentiondetection system of claim 1, further comprising: a center of the user'sface in the x-z yaw plane; and a yaw deviation corresponding to a pixeldistance between the yaw center line and the center of the user's facein the x-z yaw plane.
 10. The attention detection system of claim 9,further comprising a yaw threshold that is compared to the yaw deviationto determine whether the user is facing the display device and, if so,then instructs the computer application to execute the action.
 11. Acomputer-implemented attention detection system for determining where auser's attention is focused, comprising: at least one monitor thatprovides the user with visual information; a camera disposed on themonitor that captures images of the user including a head of the user; ayaw center line defined as a line in an x-z yaw plane that bisects theuser's head into two approximately equal parts; and a facial poseestimation system that receives and uses the captured images and yawcenter line to output facial pose information containing an estimate ofa facial pose of the user to determine where the user's attention isfocused.
 12. The computer-implemented attention detection system as setforth in claim 11, further comprising a screen saver application incommunication with the monitor that receives the facial pose informationand uses the information to decide whether to start and terminate thescreen saver application on the monitor.
 13. The computer-implementedattention detection system as set forth in claim 11, further comprisinga plurality monitors and a monitor application that uses the facial poseinformation to determine which of the plurality of monitors the user isobserving.
 14. The computer-implemented attention detection system asset forth in claim 11, further comprising an instant messagingapplication that receives the facial pose information and uses theinformation to decide whether to display a message to the user on themonitor.
 15. The computer-implemented attention detection system as setforth in claim 11, further comprising a deviation angle in the x-z yawplane that indicates a yaw angle between a center of the camera and theyaw center line.
 16. The computer-implemented attention detection systemas set forth in claim 15, further comprising a threshold angle in thex-z yaw plane that is compared to the deviation angle to determine thefacial pose information.
 17. A computer-implemented method fordetermining where a user's attention is focused in relation to acomputer display device, comprising: capturing an image of the user'shead and face using a camera; calculating a centroid of the user's headand a centroid of the user's face; comparing a position of the centroidof the user's head and a position of the centroid of the user's face todetermine where the user's attention is focused in relative to acomputer display device; and instructing a computer application whetherto execute an action based on where the user's attention is focused. 18.The computer-implemented method as set forth in claim 17, whereincomparing further comprises: calculating a deviation between thecentroid of the user's face and the centroid of the user's head; andcomparing the deviation to a threshold to determine where the user'sattention is focused relative to the computer display device.
 19. Thecomputer-implemented method as set forth in claim 18, furthercomprising: determining that deviation is greater than the threshold andthat the user's attention is not focused on the computer display device;and causing the computer application to perform background computationwhile the user's attention is not focused on the computer displaydevice.
 20. The computer-implemented method as set forth in claim 18,further comprising: determining that deviation is less than thethreshold and that the user's attention is focused on the computerdisplay device; and causing the computer application to display amessage to the user on the computer display device.